This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The principal aim is to understand the structural basis for vesicle trafficking pathways in living cells by the Rab family of small GTPases. Eukaryotic cells are a complex and dynamic system of subcellular compartments that are organized and regulated by small GTPases. Rabs comprise the largest member of the Ras superfamily with nearly 70 genes, each localized to a distinct subcellular compartment in the active (GTP-bound) state. Despite a common fold, Rabs recruit their cognate effector proteins with exquisite specificity. Effectors are typically modular, with a Rab-binding domain (RBD) and additional domains that exert biological effects such as organelle structure and inheritance, as well as vesicle formation, transport, docking and fusion. The population of effector proteins number in the hundreds, the vast majority with unknown three-dimensional structures. Further complexity has emerged with the identification of divalent effectors that can simultaneously engage two Rabs, thereby linking various trafficking pathways and subcellular compartments. Our research programme is to understand the molecular basis for vesicle dynamics by visualizing Rab-effector complexes using X-ray crystallography.